Electronic battery tester

ABSTRACT

A microprocessor couples to a voltage sensor through an analog to digital converter. The voltage sensor is adapted to be coupled across terminals of a battery. A small current source is also provided and adapted to be coupled across the terminal to the battery. The current source is momentarily applied to the battery and the resulting change in voltage is monitored using the microprocessor. The microprocessor calculates battery conductance based upon the magnitude of the differential current and the change in voltage and thereby determines the condition of the battery.

[0001] The present invention claims priority to Provisional ApplicationSer. No. 60/035,312, filed Jan. 13, 1997 and entitled “ELECTROINCBATTERY TESTER.”

BACKGROUND OF THE INEVNTION

[0002] The present invention relates to battery testing devices. Thepresent invention is particularly applicable to a technique formeasuring conductance of a battery in which a small resistive load ismomentarily placed across the battery and the change in voltage ismonitored.

[0003] Chemical storage batteries, such as lead acid batteries used inautomobiles, have existed for many years. In order to make optimum useof such a battery, it is very desirable to test the battery to determinevarious battery parameters such as state of charge, battery capacity,state of health, the existence of battery defects.

[0004] Various techniques have been used to measure battery parameters.For example, hygrometers have been used to measure the specific gravityof a battery and simple voltage measurements have been used to monitorthe voltage of the battery. One battery testing technique which has beenpopular for many years is known as a load test in which a battery isheavily loaded over a period of time and the decay in the battery outputis monitored. However, such a test is time consuming and leaves thebattery in a relatively discharged condition. Further, such a testermust be made relatively large if it is to be used with large batteries.

[0005] A much more elegant technique has been pioneered by Midtronics,Inc. of Burr Ridge, Ill. and Dr. Keith S. Champlin in which batteryparameters are determined based upon a measurement of the battery'sconductance. This work is set forth in, for example, the followingpatents issued to Champlin: U.S. Pat. No. 3,873,911; U.S. Pat. No.3,909,708; U.S. Pat. No. 4,816,768; U.S. Pat. No. 4,825,170; U.S. Pat.No. 4,881,038; U.S. Pat. No. 4,912,416; U.S. Pat. No. 5,140,269; U.S.Pat. No. 5,343,380; U.S. Pat. No. 5,572,136; and U.S. Pat. No. 5,585,728and the following patents assigned to Midtronics, Inc., U.S. Pat. No.5,574,355 and U.S. Pat. No. 5,592,093.

[0006] However, there is an ongoing need to refine battery testingtechniques, improve their accuracy and improve the types of applicationsin which they may be successfully employed.

[0007] SUMMARY OF THE INVENTION

[0008] A microprocessor couples to a voltage sensor through an analog todigital converter. The voltage sensor is adapted to be coupled acrossterminals of a battery. A small current source is also provided andadapted to be coupled across the terminal to the battery. The currentsource is momentarily switched on to provide a current (which may be acurrent drop) through the battery and the resulting change in voltage ismonitored using the microprocessor. The microprocessor calculatesbattery conductance based upon the magnitude of the current and thechange in voltage. These techniques are employed to overcome noise fromnoise sources which may be coupled to the battery during the batterytest.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a simplified electrical schematic diagram of a batterytester in accordance with the present invention.

[0010]FIG. 2 is a simplified electrical schematic diagram of a portionof sense circuitry shown in FIG. 1.

[0011]FIG. 3 is a simplified electrical schematic diagram of a portionof sense circuitry shown in FIG. 1.

[0012]FIG. 4 is a simplified electrical schematic diagram of a portionof sense circuitry shown in FIG. 1.

[0013]FIG. 5 is a timing diagram showing various signals duringoperation of the circuitry of FIGS. 1 through 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIEMENTS

[0014] It has been discovered that measuring battery conductance of astorage battery connected to noise sources is a particularly difficultproblem. Such noise sources include the charging system and variouselectronics in an automobile, for example, or other types of chargingsystems and electronics which may be connected to storage batteries.These noise sources interfere with the battery test. The presentinvention includes a number of techniques to overcome the limitationsimposed by such noise.

[0015]FIG. 1 is simplified block diagram of a battery tester 10 inaccordance with the present invention coupled to an electrical system 4.Electrical system 4 is an model which includes a charge signal noisesource 6 and a load signal noise source 8. These sources could be, forexample, the load and charger of an automobile or a uninterruptablepower system (UPS)

[0016] Battery tester 10 determines the conductance of battery 12 inaccordance with the present invention and includes test circuitry 16.Circuitry 16 includes a current source 50 (which comprises, for example,a resistance R_(L)), sensor circuitry 52, analog to digital converter 54and microprocessor 56. In one preferred embodiment, microprocessor 56comprises a Motorola MC 68HC705C8P. Sensor circuitry 52 is capacitivelycoupled to battery 12 through capacitors C1 and C2 and has its outputsconnected to a multiplexed or input of analog to digital converter 54.A/D converter 54 is also connected to microprocessor 56 which connectsto system clock 58, memory 60, output 62 and input 66. Output 62comprises, for example, a display and input 66 may comprise a keyboard,RF link, bar code reader, etc.

[0017] In operation, current source 50 is controlled by microprocessor56 using switch 100 which may comprise, for example, a FET. Currentsource 50 provides a current I in the direction shown by the arrow inFIG. 1. In one embodiment, this is a square wave or a pulse. The voltagesense circuitry 52 connects to terminals 22 and 24 of battery 12 tocapacitors C1 and C2, respectively, and provides an output related tothe voltage difference between the terminals. Sense circuitry 52preferably has a high input impedance. Note that circuitry 16 isconnected to battery 12 through a four point connection technique knownas a Kelvin connection. Because very little current flows throughcircuitry 52, the voltage drop through its connections to battery 12 isrelatively insignificant. The output of circuitry 52 is converted to adigital format and provided to microprocessor 56. Microprocessor 56operates at a frequency determined by system clock 58 and in accordancewith program instructions stored in memory 60.

[0018] In general, microprocessor 56 determines the conductance ofbattery 12 by actuating switch 100 to apply a current pulse with currentsource 50. The microprocessor determines the change in battery voltagedue to the current pulse using circuitry 52 and analog to digitalconverter 54. The value of current I generated by current source 50 ismeasured by measuring the voltage drop across resistance R_(L) usingamplifier 102. Microprocessor 56 calculates the conductance of battery12 as follows: $\begin{matrix}{{Conductance} = {G = \frac{\Delta \quad I}{\Delta \quad V}}} & {{Equation}\quad 1}\end{matrix}$

[0019] where ΔI is the change in current flowing through battery 12 dueto current source 50, and ΔV is the change in battery voltage due toapplied current ΔI. The relative conductance of battery 12, as discussedwith respect to FIG. 2, is calculated using the equation:$\begin{matrix}{{{Relative}\quad {{Conductance}(\%)}} = {\frac{G_{measured}}{G_{reference}} \times 100}} & {{Equation}\quad 2}\end{matrix}$

[0020] where G_(measured) is the battery conductance in accordance withEquation 1 and G_(reference) is a reference conductance value receivedthrough input 66 and stored in memory 60. Generally, this referenceconductance is determined based upon the type and characteristics ofbattery 12. Microprocessor 56 can also operate using impedancemeasurements by inverting Equations 1 and 2. The relative conductancemeasurement may then be output using data output 62 which may comprise,for example, a display, meter, data link, etc.

[0021] The measurement of conductance in a noisy environment usingcircuitry 16 may be accomplished by maintaining a relatively shortconnection of resistance R_(L) across battery 12 and measuring theresultant small voltage drop. The DC voltage drop across the battery isa minimum of 2 volts and the absolute voltage drop across the batterymay be any value. Sense circuitry 52 preferably has a relatively largegain which is saturated if circuitry 52 is directly coupled to battery12. Therefore, capacitors C1 and C2 are provided to capacitively coupledcircuitry 52 to battery 12.

[0022]FIG. 2 is a simplified electrical schematic diagram 110 of aportion of sense circuitry 52 shown in FIG. 1. Circuitry 100 includesdifferential amplifier 112 having an inverting input connected toterminal 22 of battery 12 through capacitor C1 and resistors 114 and 116having values of 10 KΨ and 40.2 KΩ. The non-inverting input of amplifier112 connects to terminal 24 through capacitor C2 and resistors 118 and120 having values of 10 KΩ and 40.2 KΩ, respectively. The non-invertinginput of amplifier 112 connects to electrical ground through resistor122 having a value of 1 MΩ feedback is provided from the output ofamplifier 112 through resistor 124 having a value of 1 MΩ. Capacitors C1and C2 have values of 0.1 μF and are ground through resistors 126 and128 which have a value of 1 MΩ. Low impedance path resistors 130 and 132have values of 1 KΩ and are selectively coupled to capacitors C1 and C2through switches 134 and 136, respectively. Switches 134 and 136 maycomprise, for example, FETs which are controlled by microprocessor 56.

[0023] In order to make accurate AC transient measurements, it isnecessary that the bias voltage across the input coupling capacitors C1and C2 remains relatively constant. This is facilitated by usingrelatively large capacitor values for C1 and C2 and employing coupled toa high input impedance circuit for circuit 52. However, a significantdrawback to the high impedance is that a relatively long time isrequired for the amplifier to stabilize to a quiescent operating pointwhen the tester is first started or relocated to a different battery.Resistors 130 and 132 provide a relatively low impedance path toelectrical ground when switches 134 and 136, respectively, are actuatedby microprocessor 56. Preferably, the switches 134 and 136 are actuatedjust prior to measurements to thereby quickly establish the operatingpoint of the system. A further advantage of application of the lowimpedance paths during a non-test interval is that they allow quiescentoperating points that are elevated (or depressed) due to system noise,thereby placing no practical limit on the amount of low frequency noisethat can be rejected.

[0024] Another source of inaccuracy due to noise in the system is thevariability in the voltage bias at the inputs of capacitors C1 and C2which arises due to the inductive coupling of the pulse generated bysource 50 to the voltage sense leads which couple circuitry 52 tobattery 12. This causes relatively large voltage spikes in theconnection leads which could damage the sense circuitry leading toinaccurate readings. Diode pairs 152 and 154 are provided as inputprotection devices to eliminate this and exasterbate this problem bytying one side of capacitor C1 and C2 to a power supply rail through anextremely low impedance path (the forward diode direction). In order toovercome this problem, switches 160 and 162 are provided whichselectively as shown in FIG. 3 couple capacitors C1 and C2 to resistors114 and 118, respectively. Switches 160 and 162 may comprise, forexample, FETs which are controlled by microprocessor 56. Microprocessor56 controls switches 160 and 162 to provide an open circuit during theoccurrence of any voltage that exceeds the value of the power supplyrails. Leakage is only about 1 nanoamp. This allows capacitors C1 and C2to “free wheel” during a voltage spike with no resultant in charging.

[0025] Another aspect of the invention includes the determination of thequiescent operating point of the battery voltage during application ofthe current pulse from source 50. It is desirable to exactly determinethis applicating point. However, this is not possible because thecurrent pulse has changed the operating point by an amount inverselyproportional to the conductance. Additionally, the quiescent pointvaries according to the AC or DC noise which is present on the system.The present invention estimates the quiescent operating point during thecurrent pulse by taking samples before and after the current pulse andaveraging the difference. FIG. 4 is a simplified electrical schematicdiagram of circuitry 180 which is part of circuitry 52 shown in FIG. 1.Circuitry 180 includes circuitry 52 as shown in FIG. 1. Circuitry 180includes three sample and hold elements 182, 184 and 186 which couplesto amplifier 112 shown in FIGS. 2 and 3. Additionally, sample and holdcircuits 182 through 186 receive control signals S₁, M, and S₂ frommicroprocessor 56. The output from amplifier 102 is also shown connectedto analog to digital converter 54. Analog to digital converter 54includes a multiplex input which is controlled by MUX line frommicroprocessor 56 to select one of the inputs from amplifier 102 orsample and-hold circuits 182 through 186.

[0026]FIG. 5 is a timing diagram showing operation of the circuitry inFIGS. 1 through 4. Signal S₁ is applied by microprocessor 56 to sampleand hold circuit 182, signal M is applied to sample and hold circuit 184and signal S₂ is applied to circuit 186 shown in FIG. 4. Signal S₁₀₀controls switch 100 shown in FIG. 1. The READ I signal couples analog todigital converter 54 to amplifier 102 to thereby read the voltage dropacross resistance R_(L). The S_(C) signal controls switches 160 and 162shown in FIG. 3. The READ OFFSET signal controls analog to digitalconverter 54 to initially read offsets from sample and hold 182 through186. The read ΔV signal controls reading of the sample and hold circuits182 through 186 with the A/D 54 following a measurement cycle. Duringoperation, the values of the three sample and holds are initiallylatched using the first pulse shown in signals S₁, M, and S₂. Duringthis initial reading, switches 160 and 162 are open such that thevoltages V⁰ _(S1), V⁰ _(M), V⁰ _(S2) present on these latches constituteoffset values. These offsets are stored in memory 60 and subtracted fromsubsequent voltage measurements by microprocessor 56 to thereby reduceerrors. At time t₁ switches 160 and 162 are closed by signal S_(C) andsample and hold circuit 182 is again latched using signal S₁ to storethe first measured voltage V₁. At time t₂, current I is applied tobattery 12 by closing switch 100 with signal S₁₀₀. After about 150 μS,the READ I is used to control A/D converter 54 to read the voltageoutput from amplifier 102. At time t₄, sample and hold circuit 184 istriggered by signal M to store the current voltage V_(M) across battery12. At time t₅, the current I is removed from battery 12 and after asettling period of approximately 200 μS, sample and hold circuit 186 istriggered by signal S₂ to store V₂. At time t₇, the A/D converter 54 toconvert the voltage difference of the sample stored in circuits 182 and186. In various embodiments, this difference may be determined usinganalog subtraction techniques or digital subtraction usingmicroprocessor 56. The change in voltage of the battery due to appliedcurrent I is then calculated using the formula: $\begin{matrix}{{\Delta \quad V} = {\frac{\left\lbrack {\left( {V_{1} - V_{1}^{0}} \right) + \left( {V_{2} - V_{2}^{0}} \right)} \right\rbrack}{2} - \left( {V_{M} - V_{M}^{0}} \right)}} & {{Equation}\quad 3}\end{matrix}$

[0027] G is then determined using the formula: $\begin{matrix}{G = \frac{\left( {V_{M} - V_{M}^{0}} \right)/R_{L}}{\Delta \quad V}} & {{Equation}\quad 4}\end{matrix}$

[0028] As can be seen in Equations 3 and 4, the offset values V⁰ ₁, V⁰ ₂and V⁰ _(M) are subtracted from the measured values to thereby removeany systems offsets.

[0029] Another source of errors in measurement in noisy environments isdue to lumped sum non-linearities in the circuit. In general, theequation for conductance is G=I/V, where G represents the conductance inmhos, I represents the current differential in amps and V represents thevoltage differential in volts. Non-linearities in circuit 16 may cause asmall offset component in the measured value of V. This offset may bedetermined during manufacture or during later calibration of circuitry16 by forcing the input to circuitry 16 to 0 volts and measuring theresultant voltage. This voltage value (X) is stored in memory 60 andused to modify the equation for conductance by subtracting the offsetfrom all measurements G=I/(V−X).

[0030] In another aspect of the invention, non-linearities in circuitry16 are compensated or “linearized” using a second order polynomialequation. Such non-linearities may be due to many factors includingcabling, PCB layout, magnetic effects, etc. The polynomial is determinedby measuring a plurality of calibrated standards using an uncalibratedtester 16 and the resultant data is fit to a curve using curve fittingtechniques. For example, Table 1 is a series of measurements of sevendifferent test cells having known voltage and conductance values by abattery tester prior to such calibration: TABLE 1 MEASURED ACTUAL CELLVOLTS MHOS MHOS % ERROR 1 4.40 648 800.73 +23.57 2 4.40 1080 1333.33+23.46 3 4.42 1638 2000.16 +22.11 4 4.42 2194 2665.10 +21.47 5 4.42 33414000.00 +19.72 6 4.44 5107 6001.68 +17.52 7 4.44 6968 7995.52 +14.75

[0031] Using a least squares curve fitting technique, a quadraticequation of the form: $\begin{matrix}{G_{actual} = {\text{1,34894810}^{- 1} + {1.245607G_{measured}} - {1.40414210^{- 5}G_{measured}^{2}}}} & \text{Equation~~5}\end{matrix}$

[0032] Equation 5 can be used to calibrate the measured value of mhos.The three constants in Equation 5 are stored in memory 60 for use bymicroprocessor 56.

[0033] Another technique of the present invention to overcome problemsassociated with noise includes employing statistical algorithms inmicroprocessor 56. Amplifier 12 is instantly able to take readings atany point, regardless of prior disturbance of the quiescent operatingpoint due to noise, in other words, quiescent disturbances do notrequire a long “settling period” following the disturbance beforeanother reading can be taken. If the noise signal remains linear andcontinuous, readings can be taken during the noise signal itself.However, difficulties arise in very high noise environments, where thenoise is of large value, and not linear or continuous (for example, UPSswitching currents). This “impulse” noise present during the measurementperiod causes incorrect values to be recorded for that sample, eventhough they do not affect the ability of the amplifier to take anothersample immediately following it. Noise pulses of particular concern arehigh amplitude, short duration, low frequency (360 Hz, for example)spikes. Since the measurement period is short (200 microseconds),circuit 16 can take a large number of measurements in a short period oftime. In doing so, there is a high incidence of samples containing thecorrect value of conductance, and a lower number of samples containingcorrupted data. Microprocessor 56 determines the median or mean valuesover a large number of samples and is thereby able to intelligentlydecode the correct value from the scattered measured data.

[0034] Although the present invention has been described with referenceto preferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. An apparatus for measuring conductance of abattery, comprising: a positive sense electrical connection adapted tocouple to a positive terminal of the battery; a negative senseelectrical connection adapted to couple to a negative terminal of thebattery; a positive current electrical connection adapted to couple tothe positive terminal of the battery; a negative current electricalconnection adapted to couple to the negative terminal of the battery; acurrent source; a current switch selectively coupling the current sourcebetween the positive current electrical connection and the negativecurrent electrical connection; a first capacitor connected in serieswith the positive sense electrical connection; a second capacitorconnected in series with the negative sense electrical connection; adifferential amplifier having a first input, a second input and adifferential output; a first switch selectively coupling the first inputof the differential amplifier to the first capacitor; a second switchselectively coupling the second input to the differential amplifier; andcontrol circuitry coupled to the current switch, the first switch andthe second switch selectively supplying the current source to thebattery by actuating the current switch and maintaining at least amomentary disconnection between the battery and the amplifier by closingthe first and second switches; the control circuitry responsivelydetermining battery conductance as a function of differential currentflow through the battery due to the applied and differential voltagedrop across the battery based upon the differential output.
 2. Theapparatus of claim 1 wherein the current source comprises a resistance.3. The apparatus of claim 1 wherein the control circuitry provides anoutput related to relative battery conductance as a function of thebattery conductance and a reference battery conductance.
 4. Theapparatus of claim 1 including: the third switch selectively couplingthe first input of the differential amplifier to a reference; andwherein the control circuitry actuates the third switch to momentarilycouple the first input of the differential amplifier to the referenceprior to determining battery conductance.
 5. The apparatus of claim 1wherein the control circuitry determines battery voltage prior tomeasuring current flow (V₁), during a measurement of current flow(V_(M)) and subsequent to measuring current flow and calculates changein voltage (ΔV) as a function of V₁, V_(M) and V₂.
 6. The apparatus ofclaim 5 including a first sample and hold circuit for sampling V₁, asecond sample and hold circuit for sampling V_(M) and a third sample andhold circuit for sampling V₂.
 7. The apparatus of claim 1 wherein thecontrol circuitry subtracts an offset voltage in determiningdifferential voltage drop.
 8. The apparatus of claim 7 wherein theoffset voltage is measured prior to determining battery conductance. 9.The apparatus of claim 1 including a memory and wherein the controlcircuitry compensates battery conductance as a function of an equationstored in the memory.
 10. The apparatus of claim 9 wherein the equationcomprises a polynomial of the form: G _(compensated) =a+b G _(measured)+c G _(measured) Where a, b and c are stored in the memory.
 11. Theapparatus of claim 1 wherein the control circuitry determines batteryconductance as a statistical function of a plurality of differentialcurrent flow measurements and differential voltage measurements.